1. Field of the Invention
The present invention relates to a dual band antenna, more particularly to an external twin-Vee-type dual band antenna.
2. Description of the Related Art
External antennas are designed for high gain since they are mainly used to improve reception of wireless signals by wireless devices. However, omni-directionality of radiation pattern of the external antenna is compromised in achieving the high gain of the antenna. Therefore, the external antennas generally have high gain and low omni-directionality.
FIG. 1 and FIG. 2 are schematic diagrams illustrating opposite sides of a conventional omnidirectional antenna 100 that has open-loop dipole antennas cascaded for boosting gain. The conventional omnidirectional antenna 100 includes an antenna substrate 1 having a front surface and a rear surface opposite to the front surface. A first feed-in portion 10a, a first metal line 11 and a first radiator unit 20 are disposed on the front surface of the antenna substrate 1. A second feed-in portion 10b, a second metal line 12 and a second radiator unit 30 are disposed on the rear surface of the antenna substrate 1.
Each of the first metal line 11 and the second metal line 12 is increased in width for improving impedance matching of a corresponding one of the cascaded first and second radiator units 20, 30. Nevertheless, increasing the widths of the first and second metal lines 11, 12 will result in reduced spacing between the first metal line 11 and the first radiator unit 20, and between the second metal line 12 and the second radiator unit 30. Correspondingly, signals transmitted through the metal lines 11, 12 may be coupled electromagnetically to the first and second radiator units 20, 30, which in turn affects impedance matching between the first and second radiator units 20, 30, and limits bandwidth of the conventional omnidirectional antenna 100. However, if spacings between the first metal line 11 and the first radiator unit 20 and between the second metal line 12 and the second radiator unit 30 are increased to result in reduced coupling thereamong, directionality of the antenna 100 will be increased.